Phase Linear - Fork in the Road

Hiya good people.

I inherited a Phase Linear P3800 Preamp and a DRS 900 Amplifier from a friend a few years ago, both in working condition. Aside from the amp being insanely heavy and running hot as an iron and the preamp having limited and outdated connectivity options, they've served me pretty well. I don't know much about stereo components, so I don't have a lot to compare them to, but the system was big, loud, lots of headroom, fairly musical EQ, etc.

The other day, the system started emitting a harsh, steady buzzing noise - not super loud but definitely at "something's broken" level.

I've isolated the problem down to the DRS 900 Amp, and now I'm at the proverbial fork in the road which I'm hoping you can help me navigate.

1) Is it worth repairing? I'm assuming it's going to be at least $300 when all is said and done, maybe more, probably not much less. Obviously there are a lot of variables here - I'm looking for your subjective take. If you weren't going to fix it yourself, what would you do?
2) If no, what's the best path to offload these components - either sell, or in trade for a decent quality amplifier with some digital connectivity.
3) If it is worth repairing, where in the Brooklyn/NYC area should I take it?

Thanks for any and all advice and insights!

- Lucas

 

The diagnosis will be free: It is assuredly failure of the power supply capacitors; all 70s and 80s Carver company products are due. They are called filtering capacitors because they filter out the AC ripple.

Big cans: 2 x 6800uF/100V; 2 x 4700uF/63V. There's only two amp voltage rails (unlike later Carvers with three). Your bill should be under $40 for parts. Values can be upsized by up to 20%.

The capacitors seem hidden under a flipped circuit board in web pics (and these amps weren't big sellers), so I don't know how hard finding ones in the original dimensions will be. Basic replacement, free service manual available, but you won't need an experienced Carver amp tech to do the work.




The amps don't have the cache of either earlier Phase Linear amps or Carver products since the company was running on empty by then. It's worth fixing, it's under-rated both literally and in the esteem of others. From a generation where ridiculously low THD and bandwidth that could almost power an AM radio station were considered winning the spec war.

 

Beefier than I imagined, never saw one under the hood. Big tranny, 16 output transistors, and decent heat sinking. Never would've imagined. Never heard one btw, but am a fan of the originals, 400 and 700 amps. Once reworked with new electronics and made fully complimentary, they are really nice sounding.

 

Ok, wow - thank you so much for the detailed response! So basically, find the caps, discharge them, remove and replace with caps that have identical values and are the same size and connector type? Am I missing anything?

I assume the preamp is not as integral to the sound and sonic value of the system?

Thanks again - very helpful.

 

...and install them with the correct polarity, or else you get "kaboom!"

It may take a lot of heat, like a 75W solder gun, to get them loose because of the mass of the capacitor and ground plane traces.

The preamp is part of the sound and audio quality, and you've got a matched component set, but each component can also work on its own in a heterogeneous system.

 

Interesting guts, there is a lot more than what I've expected. Do these have the same problem as earlier PL that could potentially send DC to the speakers?

 

That's the failure mode of just about any solid-state amp. The output transistors don't fail open, they fail shorted to the voltage rail. There does seem to be DC protection/sensing on the speaker output relay circuit.

 

Update...after a bit of wrangling I got the caps out. I've attached a photo of them with values and sizing, as well as the PCB I took them out of.

I think I can find suitable replacements for the smaller caps. Do these look like a good match?
https://www.mouser.com/ProductDetail/Nichicon/UVR1J472MRD6?qs=lHJjkNTO/5r7Pv8Bx%2B9IAQ==
They're not 3 pin but I wouldn't think that would be a problem, since the third pin is just for support and I almost never move the amp around.

I can't seem to find any available replacements that fit the specs AND the physical size requirements of the larger caps. I found caps w/ the right values but they are too large to fit in with the components on the PCB. And the one seeming 'direct match' [the 5-pin version of this one] is only available in bulk, and I don't need 128 units at $1700! I chatted w/ with reps at both Mouser and Digikey, after getting no matches via their search functions. Was basically told "sorry, we don't have what you are looking for".

I'm also a little confused how the polarity on the caps seems to be opposite what the markings on the PCB board are. I've indicated polarity on the caps, according to the markings - but the negative pins correspond to the PCB holes with the "+" next to them. Or is it an "x", as in 'x' marks the spot where the negative goes"? That seems...weird. Anyway, I am bumping up against my very limited knowledge and could use some guidance if anyone has the wherewithal! Thanks very much in advance!

 

For the large caps with that pinout, the 5-pin series is United Chemi-con VQT in 40mm diameter. Discontinued. According to Octopart, "Lucentia Tech" has ESMH101VQT123MB80T (40x80, 12000uF 100V) in stock (with other unlikely ones), but that site looks sketchy; address is a warehouse that truckers complain about.

Perhaps the closest part, with 22.5 lead spacing instead of 25mm, two additional support pins instead of three. Holes will need to be lengthened, and I would glue the cap to PCB with clear RTV:
ESMH101VND103MA63T United Chemi-Con | Capacitors | DigiKey

Quality part with 18000 hours instead of 2000 hours:
ALC70C682KP250 KEMET | Capacitors | DigiKey

Were it my repair, I might make a riser card for each with 2x Kemet ALC70A332DC100 - 3300uF 100V 15000hr (35x37). Manufacturers these days tend to parallel smaller values also.

 

Thanks for the suggestions.
That first link is for a cap at 10000µF 100V – wrong values for either cap and outside the range of acceptable deviation, right?
The second link is for a cap w/ the right values but it won't fit inside the amp enclosure at 50mm dia. / 107mm length.
As for the third suggestion – I'm guessing a riser card lets you use 2 caps to connect to one spot on the PCB. Again, there's just not enough room in there - it's very tight as-is.
I think I may just call it a day and eBay this monster as-is for parts. The OCD perfectionist in me wants to dig in and make it right but their are approximately 43 other projects that are all "Hey! What about us?!?!"

 

I have two of those very Kemet 3300uf/100 v caps among some other caps on their way to me from Digi-Key as my Fisher console's hybrid power amplifier has some recapping it needs, one failed open. A good recommendation.

 

The 10000uF should be fine, I didn't make a mistake there. Lower ESR and more reserve. There's also two-pin 10mm pin spacing United Chemicons in stock.

Two 35mm diameter caps on edge: 70mm length of an offset 80mm riser card made of perfboard. I'd add an angle bracket soldered to riser and bolted to board. You depicted the side capacitors mount to; I see no obstacles.

If the hum is at idle, there is a chance it is just a low voltage rail 63V cap that failed. The amp doesn't employ the high rail except for loud peaks, although I doubt the amp would be noise-free with one of those out either.

 

So 10000uf 100V to replace the large caps should work? I was under the impression it was ok to go bigger w. voltage value but that the cap value should stay fairly close to what was in there. Happy to be wrong on that if it solves my problem!

Will these work for the 4700uf/63V caps?
UVR1J472MRD6 Nichicon | Capacitors | DigiKey

I'm definitely not going to do any custom build in there - just can't handle the learning curve right now, but I appreciate the suggestion.

 

If you don't have a variac, google up "poor man's variac" (basically a power cable with a light bulb in series with the hot side of the 120V line) and employ one the first time you start it up. It'll cost you less than $5 to build, you'll have it forever, and it will save you from any mistakes.

 

Seems like a good choice, others are much more expensive or have leads even more closely spaced than original.

In an unregulated amp, bigger power cap is better, up to the point where there is just no ripple benefit possible for the DC load, or you are sacrificing opportunity for instead higher surge voltage, lifespan, or temperature (105C better) ratings.

The concern with larger capacitor values is turn-on inrush current through the transformer or rectifiers, but these cap values are not the largest seen in amps. The bridge rectifier here is rated 2.5X the fuse. Capacitor tolerance is often +50% or more, so a design should accommodate such.

 

Ok - I got the replacement caps for the power supply and even a few smaller replacement caps for some that looked a bit cooked. I also noticed that there were 2 small parts (D11 + D12)that look like they got torched. Picture is attached. According to the service manual, they are "1N758: 10V" diodes, which are 10v, 400mW Zener diodes. Can I safely replace them with 10v, 500W Zener diodes? I tried to test them with a multimeter but my skills/knowledge are very, very limited and my readings were inconclusive. Thanks!

 

I assume you mean 500 mW diodes (500 watt ones would be pretty big) and those will be fine, it's the voltage that counts. I'd replace those just looking at the burned board, but if you do want to test them for whatever reason, unsolder one end and look on your meter for a range with a diode symbol - most digital multimeters will have one. Check in both directions, you should see about .6xxx in one direction and nearly open in the other if they are OK. Then unsolder the other end and replace them.

 

Thanks - and yes, I meant 500 mW! Appreciate the help!

 

The diodes are probably fine. The heat they dissipate while regulating voltage has just discolored the PCB over 30 years of operation.

 

Just wanted to update this thread - I replaced the power supply caps and a few others that looked sketchy. Amp powers up and....no hum or buzz! I'm not celebrating yet - need to see how things play out over the next week or so - but so far, so good.

Thanks so much to harby, and everyone else who replied, for the insights and recommendations. I really appreciate the help!